What I know of E5.5 and clustering:
Exchange Server 5.5 Enterprise Edition includes support for Microsoft Cluster Server version 1.0. Clustering your Exchange environment ensures that your messaging environment remains uninterrupted even if one server fails. When one of the clustered servers fails, the other server takes on that server's load with no impact to the users. An Exchange cluster consists of two Exchange servers that share one or more common disk drives, an IP address, a network name, and Exchange Server cluster resources. All network requests to the Exchange cluster are sent to a "virtual server" that forwards the requests to the active server in the cluster. By clustering your Exchange servers, you are adding yet another level of fault tolerance to ensure the availability of your messaging system.
It goes without saying that in order to have Exchange on Cluster Server, the first thing you have to do is install and configure Cluster Server. Once you've installed and configured Cluster Server, you're ready to run the Exchange Server setup program on the active node. The setup program will copy the files to the active node's system32 directory and to the clustered drive, and it creates resources in the Exchange Server's cluster resource group. After Exchange has completed setup, you should run the Exchange performance optimizer. Next, run the setup program on the secondary node, being careful to select UPGRADE NODE. Setup will copy the files to the system32 directory on the secondary node, where it also creates the Exchange services. (Notice that you can't run Performance Optimizer on the secondary node.) Remember, when you install any new Exchange components on the primary node, you must also install it on the secondary node.
When installed on a Cluster Server, Exchange's services are set to manual start, which prevents the automatic startup of the Exchange services. The Cluster Server's resource manager starts the services in order of dependency. If you need to manage the services on an Exchange server in a clustered server environment, you should only use the Cluster Administrator program, not the services control panel or the Net Stop command. Keep in mind, though, that you should fail over the Exchange server cluster group prior to stopping the services.